/locate/rggThe stages and duration of formation of gold mineralizationat copper-skarn deposits (Altai–Sayan folded area )I.V. Gaskov *, A.S. Borisenko, V.V. Babich, E.A. NaumovV.S. Sobolev Institute of Geology and Mineralogy, Siberian Branch of the Russian Academy of Sciences,prosp. Akad. Koptyuga 3, Novosibirsk, 630090, RussiaReceived 20 March 2009; accepted l6 November 2009AbstractGold mineralization at copper-skarn deposits (Tardanskoe, Murzinskoe, Sinyukhinskoe, Choiskoe) in the Altai–Sayan folded area is related to different hydrothermal-metasomatic formations. It was produced at 400–150 ºC in several stages spanning 5–6 Myr, which determined the diversity of its mineral assemblages. Gold mineralization associated with magnetite bodies is spatially correlated with magnesian and calcareous skarns, whereas gold mineralization in crushing zones and along fault sutures in moderate- and low-temperature hydrothermal-metasomatic rocks (propylites, beresites, serpentinites, and argillizites) is of postskarn formation. Different stages were manifested with different intensities at gold deposits. For example, the Sinyukhinskoe deposit abounds in early high-temperature mineral assemblages; the Choiskoe deposit, in low-temperature ones; and the Tardanskoe and Murzinskoe deposits are rich in both early and late gold minerals. Formation of commercial gold mineralization at different copper-skarn deposits is due to the combination of gold mineralization produced at different stages as a result of formation of intricate igneous complexes (Tannu-Ola, Ust’-Belaya, and Yugala) composed of differentiated rocks from gabbros to granites.© 2010, V.S. Sobolev IGM, Siberian Branch of the RAS. Published by Elsevier B.V. All rights reserved.Keywords: gold mineralization; skarns, copper-skarn deposits; hydrothermal-metasomatic formationsIntroductionRecent data on the isotope geology and geochronology of rocks and ores and geological data on the ore genesis gaps proved that ore deposits formed for a much longer time than was assumed earlier (Rundkvist, 1997). This is also true for commercial gold mineralization at many Cu-skarn deposits in the Altai–Sayan folded area (ASFA).Gold-containing Cu-skarn deposits are widespread in many ore districts of the ASFA: Gorny Altai (Sinyukhinskoe,Murzinskoe, Choiskoe), Kuznetsk Alatau (Natal’evskoe, Fe-dorovskoe), Gornaya Shoria (Maisko-Lebedskoe), and Tuva (Tardanskoe, Khopto). Most of them are commercial deposits (Fig. 1).Skarn formation processes at these deposits were related to the Early and Middle Paleozoic granitoid magmatism in the Tannu-Ola (eastern Tuva), Yugala (Sinyukha, northeastern Altai), and Ust’-Belaya (northwestern Altai) intrusive com-plexes (Gusev, 2007; Shokalsky et al., 2000). Formation of commercial gold mineralization was a longer and more intricate process (Gaskov, 2008). In most part of these deposits, gold mineralization is the product of multistage ore process, which is characterized by different mineral composi-tions and spatial occurrences. Almost all these deposits bear gold mineralization spatially and genetically related to skarns and aposkarns in assemblage with magnetite and sulfides (Korobeinikov and Matsyushevskii, 1976; Korobeinikov and Zotov, 2006; Korobeinikov et al., 1987; Vakhrushev, 1972)and gold mineralization isolated from skarns and represented by sulfide-containing (pyrite, chalcopyrite, bornite, chalcocite)hydrothermal products of moderate-temperature assemblage in crushing zones (Shcherbakov, 1974). Often, the deposits also bear epithermal gold-containing assemblage with low-tem-perature sulfides, tellurides, and selenides usually developed at the final stage of mineral formation in rocks of different compositions, including sedimentary, igneous, and skarn (Gas-kov, 2008; Gaskov et al., 2005).The recently obtained ages of ore formation products and igneous rocks (Gaskov, 2008; Rudnev et al., 2004, 2006;Shokalsky et al., 2000) provide a new concept of the sequence of ore formation and its duration and relation with multiphasemagmatism.Russian Geology and Geophysics 51 (2010) 1091–1101*Corresponding author.E-mail address : gaskov@uiggm.nsc.ru (I.V. Gaskov)doi:10.1016/j.rgg.2010.0.0011068-7971/$-see front matter D 2010, IG M, Siberian Branch of the RAS.Published by E lsevier B.V .All rights reserved.V S. .Sabolev 9Let us dwell on the specific features of gold mineralization at particular deposits.Gold mineralization at Cu-skarn depositsThe Tardanskoe deposit is localized in the zone of the Kaa-Khem deep fault, in the exocontact part of the Kopto-Baisyut gabbro-diorite-plagiogranite massif (Fig. 2) (Korobe-inikov and Zotov, 2006; Korobeinikov et al., 1987). At the massif contact, Lower Cambrian volcanogenic-carbonate de-posits are transformed into magnesian and calcareous skarns described in detail earlier (Korobeinikov, 1999; Korobeinikov and Matsyushevskii, 1976; Korobeinikov et al., 1997). The skarn bodies are spatially close to aposkarn metasomatites bearing actinolite, tremolite, epidote, serpentine, chlorite, talc,quartz, carbonate, magnetite, and hematite.Gold mineralization at the deposit is of two types: (1) in skarn-magnetite rocks and (2) in metasomatites of linear crushing zones. These types have specific mineralogical and geochemical features.Gold mineralization in skarn-magnetite ores is widespread at the deposit. It is described elsewhere (Korobeinikov and Matsyushevskii, 1976; Korobeinikov and Zotov, 2006; Koro-beinikov et al., 1987; Kudryavtseva, 1969). Gold is spatially related to areas of sulfide mineralization, and its contents are in direct correlation with the amount of sulfide minerals.Gold-sulfide mineralization is extremely unevenly distributed and is localized at the sites of magnetite ores that underwent cataclasis as well as in magnetite microcracks and interstices.The total amount of sulfides (pyrite, chalcopyrite, bornite, and scarcer sphalerite, pyrrhotite, and arsenopyrite) is 1–3%. Gold occurs as fine thin (0.3–0.01 mm) native segregations. This is mainly high-fineness gold (820–990) (Fig. 3, a ) with impuri-ties of silver (up to 13.6%) and copper (up to 5.07%).According to Korobeinikov (1999) and Korobeinikov and Matsyushevskii (1976), the temperatures of formation of magnetite ores were 430–550 ºC, whereas the gold-sulfide assemblage and the hosting metasomatites (actinolite, tre-molite, serpentine, talc) were produced at 250–320 ºC (Gaskov et al., 2005; Vakhrushev, 1972).Gold mineralization in crushing zones is localized in steeply dipping linear tectonic structures of NW, NE, and NS strikes (Fig. 2), which develop after different rocks, including volcanosedimentary, igneous, and skarn ones. These zones reach several hundred meters in length and few tens of meters in width. The petrographic composition of these zones is di-verse and depends mainly on the composition of initial rocks that underwent transformation later. The rocks are metaso-matic, close in composition to propylites, listwaenites, talc-containing and sericite-quartz metasomatites, and beresite-like rocks. Almost each type of hydrothermal-metasomatic rocks is intimately associated with ore minerals. Though the total volume of these minerals does not exceed 3–5%, they are extremely diverse in composition and are extremely unevenly distributed. Along with sulfide minerals typical of Cu-skarn deposits (chalcopyrite, pyrite, bornite, chalcocite,digenite, sphalerite, galena), the mineralized zones of the deposit abound in tellurides—hessite (Ag 2Te), tellurobis-muthite (Bi 2Te 3), and tetradymite (Bi 2Te 2S),—and low-tem-perature Co and Ni sulfides and sulfoarsenides (Table 1). The latter have a variable composition and often consist of intermediate phases of continuous mineral series, e.g., allo-clasite(CoAsS)–arsenopyrite(FeAsS) or siegenite(CoNi 2S 4)–violarite(FeNi 2S 4).Gold occurs mainly as native fine thin (0.01–0.5 mm)disseminations in rock microcracks and as inclusions in pyrite,chalcopyrite, and bornite. The gold fineness varies over a broad range of values—from 440 to 820 (Fig. 3, b ). The lowest-fineness gold segregations are compositionally similar to electrum and have high contents of Ag (up to 54.78%) and Hg impurity (up to 3.65%).On the flanks of mineralized crushing zones, there is sometimes gold mineralization in low-temperature argillitized rocks of chlorite-kaolinite-carbonate-hydromica composition.This gold is of low fineness (no more than 600). The mainimpurities are Ag (20–66%) and Hg (up to 5.47%). The formation temperatures of sulfide-telluride assemblages andFig. 1. Schematic occurrence of gold-bearing Cu-skarn deposits in the Altai-Sayan folded area: 1, Murzinskoe; 2, Sinyukhinskoe; 3, Choiskoe; 4, Maisko-Lebedskoe;5, Fedorovskoe; 6, Natal’evskoe; 7, Tardanskoe; 8, Kopto.1092I.V. Gaskov et al. / Russian Geology and Geophysics 51 (2010) 1091–1101gold mineralization in metasomatites and argillitized rocks are within 200–75 ºC.The Murzinskoe deposit is localized at the contact of a small stock-like granodiorite body of the Ust’-Belaya gabbro-diorite complex (Fig. 4). In the exocontact zone, calcareous skarns composed of garnet, pyroxene, wollastonite, and mag-netite develop after the calcareous sandstones of the Murzinka Formation (D1-2). In the local zones, there are aposkarnFig. 2. Schematic geologic structure of the Tardanskoe deposit (compiled after the data of K.M. Kil’chichakov and L.V. Kopylova and our new data). 1–4, Lower Paleozoic deposits: 1, andesitic porphyrites and tuffs with siltstone and sandstone interbeds in the lower part of the Tumat-Taiga Formation (Cm 1tm 1); 2, quartz porphyrites with interbeds of andesitic porphyrites and limestones in the upper part of the Tumat-Taiga Formation (Cm 1tm 2); 3, limestones and calcareous shales of the Tapsa Formation (Cm 1tp); 4, Lower and Middle Silurian conglomerates and sandstones (S 1-2); 5, Quaternary deposits (Q IV ); 6, 7, Lower Paleozoic igneous rocks of the Tannu-Ola complex (γδO 1-2): 6, gabbro-diorite-plagiogranite formation; 7, small granite-porphyry and quartz diorite bodies; 8, calcareous and magnesian skarns; 9, hydrothermal-metasomatic rocks in mineralized crushing zones; 10, gold orebodies; 11, tectonic zones; 12, geologic boundaries.I.V. Gaskov et al. / Russian Geology and Geophysics 51 (2010) 1091–11011093Fig. 3. Variations in gold fineness in gold ores from skarn-magnetite bodies (a) and in ores from mineralized crushing zones (b) at the Tardanskoe deposit.Table 1. Mineral parageneses in gold-bearing ores produced at different stages and composition of host rocks at Au-Cu-skarn depositsDeposit Early aposkarn Au-sulfide mineralization in magnetite-skarn rocks Late Au-telluride-sulfide mineralization in superposed crushingzonesOre parageneses Host rocks Ore parageneses Host rocksTardanskoe Magneite (Fe3O4)Pyrite (FeS2)Chalcopyrite (CuFeS2)Bornite (Cu5FeS4)Sphalerite (ZnS)Pyrrhotite (FeS)Arsenopyrite (FeAsS)Gold (Au)Magnesian skarns (pyroxene +fassayite + phlogopite +pargasite + forsterite + spinel).Calcareous skarns (pyroxene +garnet + epidote +wollastonite + skapolite).Aposkarn serpentine andserpentine-chlorite rocksCobaltite (CoFe)AsSGlaucodot (Co,Fe)AsSSiegenite (CoNi2S4)Violarite (FeNi2S4)Hessite (Ag2Te)Gold (Au)Propylites, listvaenites, talc-serpentine-containing andsericite-quartz metasomatites,and argillitized rocksMurzinskoe Magnetite (Fe3O4)Chalcopyrite (CuFeS2)Pyrite (FeS2)Bornite (Cu5FeS4)Sphalerite (ZnS)Galena (PbS)FahloreArsenopyrite (FeAsS)Clinobisvanite (BiVO4)Gold (Au)Calcareous skarns (garnet +pyroxene + wollastonite).Aposkarn metasomatic rocks(quartz + epidote + chlorite +actinolite)Cinnabar (HgS)Metacinnabarite (HgS)Bismuthine (Bi2S3)Aikinite (CuPbBiS3)Emplectite (CuBiS2)Berryite [Pb2(Cu,Ag)3Bi5S11]Naumannite (Ag2Se)Polybasite (Ag16Sb2S11)Barite (BaSO4)Gold (Au)Quartz and quartz-carbonateveins, near-vein metasomatitesof quartz-chlorite-carbonatecomposition, and argillitizedrocksSinyukhinskoe Magnetite (Fe3O4)Pyrite (FeS2)Chalcopyrite (CuFeS2)Bornite (Cu5FeS4)Chalcocite (Cu2S)Sphalerite (ZnS)Pyrrhotite (FeS)Cubanite (CuFe2S3)Gold (Au)Wollastonite, garnet-wollastonite, garnet-pyroxeneand pyroxene skarns, andaposkarn metasomatic rocks(chlorite + actinolite + calcite)Tetradymite (Bi2TeS)Siegenite (CoNi2S4)Cobaltite ((CoNiFe)AsS)Melonite (NiTe2)Wittichenite (Cu3BiS3)Hessite (Ag2Te)Petzite (AuAg3Te2)Altaite (PbTe)Clausthalite (PbSe)Gold (Au)Local zones of actinolite-chlorite-calcite-quartzcompositionChoiskoe Magnetite (Fe3O4)Pyrite (FeS2)Chalcopyrite (CuFeS2)Gold (Au)Garnet, garnet-pyroxene,garnet-wollastonite, andpyroxene-epidote skarnsTetradymite (BiTe2S)Ingodite (Bi2TeS)Joseite (Bi4TeS2)Hedleyite (Bi2Te)Tellurobismuthite (Bi2Te3)Bismuthite (Bi2S3),Native bismuth (Bi)Gold (Au)Quartz and quartz-carbonateveins and quartz-carbonate-chlorite metasomatites1094I.V. Gaskov et al. / Russian Geology and Geophysics 51 (2010) 1091–1101metasomatic rocks consisting of quartz, epidote, calcite,chlorite, actinolite, and, more seldom, tourmaline, apatite, and rodonite.Gold mineralization at the Murzinskoe deposit was earlier ascribed to gold-skarn type. But recent data have shown that only a minor part of the deposit ores — scarce postskarn sulfide mineralization spatially associated with skarn-magnet-ite bodies—can be referred to this type. Most of the commer-cial ores occur in mineralized crushing zones. They form gold-sulfide mineralization in quartz and quartz-carbonate veins and near-vein metasomatites in a 300–400 m thick zone stretching in the N-NW direction for more than 3 km (Fig. 4).The crust of weathering widespread at the deposit contains hypergene copper minerals: malachite, chrysocolla, azurite,chalcocite, coveline, and high-fineness gold.Gold-sulfide mineralization spatially associated with skarn-magnetite bodies is superposed on skarn rocks. It was produced either at the regressive stage of the skarn formation or at the postskarn hydrothermal-metasomatic stage and was accompanied by the formation of moderate- and low-tempera-ture metasomatic minerals—chlorite, actinolite, epidote, and quartz. Sulfide mineralization is unevenly distributed and occurs as veinlet-disseminated chalcopyrite, pyrite, bornite,and sphalerite. It amounts to few percent. Gold occurs as fine thin (0.5–0.01 mm) native segregations. It is mainly of high fineness (840–994) (Fig. 5, a ).In crushing zones (Fig. 4), gold mineralization was found in quartz-carbonate-sulfide veinlets and veins in hydrothermal-metasomatic rocks of quartz-chlorite-carbonate composition with kaolinite, hydromica, and adularia (argillizite formation)developing after different rocks—skarns, hornfelses, shales,siltstones, and limestones,—often beyond skarning and horn-felsing zones. The quartz veins are 0.1 to 2.0 m (on average,0.4 m) thick, of N-S strike and eastern dip. In contrast to the gold-skarn-magnetite type, this mineralization is of more complex composition. In addition to minerals typical of skarn deposits (chalcopyrite, pyrite, bornite, sphalerite, and galena),it includes fahlore, arsenopyrite (FeAsS), cinnabar (HgS),metacinnabarite (HgS), bismuthine (Bi 2S 3), aikinite (CuPb BiS 3), emplectite (CuBiS 2), berryite [Pb 2(Cu,Ag)3Bi 5S 11],naumannite (Ag 2Se), polybasite (Ag 16Sb 2S 11), scheelite (Ca 3WO 4), hematite (Fe 2O 3), clinobisvanite (BiVO 4), bariteFig. 4. Schematic geologic structure of the Murzinskoe deposit. 1, mica-sili-ceous shales (O 1); 2, sandstones, siltstones, and aleuropelites (S 1); 3, terri-genous-carbonate deposits (D 1-2): a , conglomerates, b , limestones, c , sand-stones; 4, granodiorites of the Ust’-Belaya complex (D 3); 5, altered rocks and metasomatites: a , hornfelses, b , skarns, c , quartz-tourmaline metasomatites;6, mineralized crushing zones; 7, faults: a , established, b , predicted; 8, other types of mineralization: a , Murzinka-3 (Au), b, skarn Fe.Fig. 5. Variations in the fineness of gold associated with skarn-magnetite bodies (a ) and gold from ores of mineralized crushing zones (b ) at the Murzin-skoe deposit.I.V. Gaskov et al. / Russian Geology and Geophysics 51 (2010) 1091–11011095(BaSO 4), and gold (Table 1). The content of gold in the ores varies over a broad range of values, from 0.1 to 232 ppm.This gold occurs as fine (<0.1 mm) thin segregations in assemblage with sulfides. Its fineness also greatly varies (640–840), but, compared with the first type of ores, low-fine-ness gold prevails here (Fig. 5, b ).The presence of cinnabar, sulfides and sulfosalts of Bi, Se,and Sb, and barite, predominance of low-fineness gold and electrum, and low-temperature wallrock alteration (formation of kaolinite, hydromica, and adularia) differ these ores from earlier formed ores in skarn-magnetite bodies. The gap between the skarn and ore formation processes is evidenced from the presence of basite dikes cutting the skarns, which bear superposed gold mineralization of this type. At the same time, the presence of gold–cinnabar intergrowths and fine dissemination of gold in cinnabar, presence of Hg-minerals (cinnabar, Hg-sphalerite, saucovite) in the ores, and high contents of As, Sb, and Ti (typical elements of many Au-Hg deposits) permit this mineralization to be referred to as epithermal Au-Hg type (Borisenko et al., 2006). Thermometric studies showed that the homogenization temperatures of fluid inclusions in quartz veins in the northern and central parts ofthe mineralized zone are 215–200 ºC and decrease to 160–130 ºC in the southern part.Fig. 6. Schematic geologic structure of the Sinyukhinskoe deposit (compiled by Gusev (2007) and supplemented by our data). 1, loose Quaternary deposits; 2–6, rocksof the Choya (O 1cs), Elanda (C−2-3el), Ust’-Sema (C −2us), and Upper Ynyrga (C −2vy) Formations: 2, conglomerates, 3, siltstones, 4, sandstones, 5, limestones,6, andesite-basaltic porphyrites; 7–9, rocks of the Yugala (Sinyukha) complex: 7, granites and granodiorites of the early phase (γδD 2-3), 8, granites of the late phase (γD 2-3), 9, dolerite and gabbro-dolerite dikes; 10, plagiogranites of the Sarakoksha complex (ν C −2); 11, skarns; 12, sites with gold mineralization (1, Pervyi Rudnyi (First Ore), 2, Zapadnyi (Western), 3, Faifanov, 4, West Faifanov, 5, Ynyrga, 6, Nizhnii (Lower), 7, Tushkenek, 9, Gorbunov); 13, faults.1096I.V. Gaskov et al. / Russian Geology and Geophysics 51 (2010) 1091–1101The Sinyukhinskoe deposit is localized in northeastern Altai, at the contact of the large (600 km 2) complex Sarakok-sha pluton and Cambrian volcanosedimentary strata of the Ust’-Sema Formation (Shcherbakov, 1967; Vakhrushev, 1972)(Fig. 6). According to Shokalsky et al. (2000) and Gusev (2007), this massif includes the Lower Cambrian Sarakoksha diorite-tonalite-plagiogranite complex and Lower Devonian Yugala gabbro-diorite-granite complex (Sinyukha complex (Gusev, 2003)). It is in the latter complex that the commercial mineralization of the Sinyukha ore field is localized. In the contact zone of the Sinyukha massif, skarns of different compositions are developed in horizons of carbonate rocks and tuffs. Wollastonite and garnet-wollastonite varieties are the most widespread, and garnet-pyroxene and pyroxene ones are scarcer. Near the contact with basic effusive bodies, small magnetite orebodies have been revealed among garnet-py-roxene skarns.Gold mineralization occurs mainly among wollastonite,garnet-wollastonite, and pyroxene-wollastonite skarns and is intimately associated with an assemblage of sulfide minerals.The latter are dominated by bornite, chalcocite, chalcopyrite,and pyrite, which compose ore zones in these rocks and are present in the form of nest-disseminations and stockworks. In local zones of actinolite-chlorite-calcite-quartz composition we found minor amounts of sphalerite, pyrrhotite, cubanite, and tetradymite. There are also occasional findings of rare miner-als, such as siegenite (CoNi 2S 4), cobaltite ((CoNiFe)AsS),melonite (NiTe 2), wittichenite (Cu 3BiS 3), gessite (Ag 2Te),petzite (AuAg 3Te 2), altaite (PbTe), and clausthalite (PbSe)(Table 1). The total content of sulfides does not exceed 5–10%. The sulfides are extremely unevenly distributed—from occasional dissemination to densely disseminated, almost massive ores. The composition of sulfide mineralization slightly changes with depth: Gold-chalcocite-bornite assem-blage is changed by gold-chalcopyrite one. The accumulation of gold-sulfide mineralization was accompanied by the hy-drothermal-metasomatic alteration of the host skarns with the formation of actinolite, chlorite, and calcite near ore veins and nests. Magnetite ores are poorer in gold, and sulfide-free rocks(marbles and diorite-porphyry and granite-porphyry dikes)virtually lack it.Fig. 7. Variations in gold fineness in ores from the Sinyukhinskoe deposit.Fig. 8. Schematic geologic structure of the Choiskoe deposit (compiled by Gusev and Gusev (1998) and supplemented by our data). 1–5, rocks of the Ishpa (O 1is) andTandosha (C−2-3td) Formations: 1, conglomerates, 2, siltstones, 3, sandstones, 4, limestones, 5, felsic tuffs; 6–7, granitoids of the Yugala complex: 6, granites and granodiorites of the early phase (γδD 2-3), 7, leucocratic granites of the late phase (γD 2-3); 8, granite-porphyry, diorite, and lamprophyre dikes (γδD 2-3); 9, skarns;10, gold mineralization occurrences (1, occurrence of the Central skarn deposit, 2, Pikhtovyi, 3, Smorodinovyi); 11, faults.I.V. Gaskov et al. / Russian Geology and Geophysics 51 (2010) 1091–11011097Gold often occurs in ores as native segregations in the form of hooks, fine wires, lumps, and sheets intimately intergrown with bornite, chalcocite, and chalcopyrite. Sometimes, native gold segregations are observed as fine inclusions in cracks and interstices of skarn minerals, most often, wollastonite. These gold particles are mainly no larger than hundredths of millimeter. The gold of primary ores of the Sinyukhinskoe deposit is of high fineness varying over a narrow range of values (911–964) (Fig. 7). The fineness of gold decreases to 860–870 only in its parageneses with tellurides, selenides, and rare sulfide minerals (Roslyakova et al., 1999). The main impurities in gold are silver (up to 19%) and copper (up to 1.7%). The content of Hg does not exceed 0.1%. By the formation conditions, these ores are postskarn hydrothermal,with their deposition temperatures not exceeding 350 ºC (Roslyakova et al., 1999; Shcherbakov, 1972).The Choiskoe deposit is localized 20 km northeast of the Sinyukha ore field, in the zone of contact between the Upper Cambrian terrigenous-carbonate deposits of the Ishpa Forma-tion and the Choya granitoid massif referred to the Lower Devonian Yugala gabbro-diorite-granite complex (Fig. 8). The Choya granitoid massif is small at the surface (1 × 5 km) and extends from west to east, tracing the Choya fault (Gusev,2007). The deposit abounds in dikes of dolerite porphyrites,diorites, and granite-porphyry and in rocks of the lamprophyre series—kersantites, minette, and spessartites. The zone of contact between the granitoids of the Choya massif and the horizons of limestones and terrigenous-carbonate rocks is composed of skarns, which form linear zones extending in the NE direction, like the other rocks. Most bodies are of persistent thickness, ~100 m. By composition, the skarn bodies are divided into zones of garnet, garnet-pyroxene, pyroxene,garnet-wollastonite, and pyroxene-epidote skarns. In the skarn zones and near lamprophyre bodies, poor scheelite-molybde-nite mineralization in quartz veins was established (Gusev,1998).Gold mineralization at the deposit occurs in linear tectonic zones and is not spatially associated with skarns. It develops as quartz veins and quartz-carbonate and quartz-carbonate-chlorite veinlets and nests with gold-sulfide mineralization in crushing and brecciation zones in both the skarns and the granitoids of the Choya massif (Fig. 8).The mineral composition of these objects is nearly the same—gold-sulfide and gold-telluride parageneses. A numberof rare tellurides have been revealed among the Choya deposit ores: tetradymite (BiTe 2S), ingodite (Bi 2TeS), joseite (Bi 4TeS 2), hedleyite (Bi 2Te), tellurobismuthite (Bi 2Te 3), bis-muthine (Bi 2S 3), and native bismuth (Table 1). Magnetite,pyrite, and chalcopyrite, typical minerals of Cu-skarn deposits,are extremely scarce here. The total content of sulfides does not exceed few percent. They occur mainly as fine thin dissemination and do not form large accumulations and nests.Gold in the Choya deposit ores occurs as fine inclusions in sulfide and telluride minerals in quartz veinlets and as intergrowths with ore minerals. The gold particles are hun-dredths and tenths of millimeter in size. By chemical compo-sition, the gold is divided into two groups: medium-fineness (843–880) and high-fineness (940–959); the latter is probably of exogenous nature (Fig. 9). The gold contains Ag (3–12.5 wt.%) and Hg (0–0.48 wt.%) impurities and Cu traces.The thermometric studies showed that homogenization of primary gas-liquid inclusions into liquid proceeds at 126–150 ºC in quartz and at 105–128 ºC in calcite from ore-bear-ing veins.The sequence and duration of formation of gold mineralization and its correlation with magmatism As seen from the above data, gold mineralization at all considered Cu-skarn deposits has a complex multistage for-mation history. But the same stages at different deposits ran with different intensities. For example, at the Sinyukhinskoe deposit, mainly early high-temperature mineral assemblages are widespread, whereas at the Choiskoe deposit, low-tempera-ture ones. The Tardanskoe and Murzinskoe deposits bear both early and late minerals. To elucidate the peculiarities of gold-ore formation, establish the correlation between different types of gold mineralization and magmatic activity, and evaluate the duration of ore formation, we performed Ar-Ar and U-Pb dating of different mineralization and igneous rocks from the Tardanskoe and Murzinskoe deposits.Our investigations have shown that the formation of gold mineralization at the Tardanskoe deposit lasted for a longer time than it was supposed earlier. Skarn mineralization formed at the contact of diorites with carbonate rocks as a result of the intrusion of the Kopto-Baisyut massif. Ar-Ar biotite dating of the massif yielded an age of 485.7 ± 4.4 Ma corresponding to the Early Ordovician (Table 2). The skarns at the massif contact as well as magnetite ores and gold-sulfide mineraliza-tion (pyrite, chalcopyrite, pyrrhotite, bornite, gold) spatially and genetically associated with skarn-magnetite bodies are of similar age. Gold was deposited together with sulfides, as evidenced from the direct correlation between the contents of gold and sulfides (especially chalcopyrite) and from gold inclusions in the sulfides. The formation of skarn and aposkarn mineralization was followed (with some temporal gap) by the intrusion of dike and stock-like small granitoid bodies, which is indicated by their cutting of the sulfide-bearing skarn and magnetite bodies. Ar-Ar dating of these granite bodies yielded an age of 484.2 ±4.3 Ma (Table 2).Fig. 9. Variations in gold fineness in ores from the Choiskoe deposit.1098I.V. Gaskov et al. / Russian Geology and Geophysics 51 (2010) 1091–1101。
